Boilers used to generate steam have been treated with a variety of synthetic and naturally occurring chemicals in an effort to reduce the formation of scale on heat exchange surfaces. Scale, when of a sufficient thickness, tends to reduce heat transfer and in some instances produces metal overheating. When these problems are aggravated by large scale accumulation, rupture of the heat exchange surfaces can occur causing possible injury and requiring extended down-time for repairs.
In the effort to achieve greater efficiency in industrial boilers for the production of steam for industrial energy purposes, the trend of boiler design has been toward higher and higher operating pressures and temperatures. Careful studies have been made of the heat-transfer properties of structural metals, notably steels, with especial regard for the temperature gradient across the thickness of wall of a pressure-confining heat-transfer boiler member, typically a tube. Commonly, such tubes operate with one surface adjacent a heat source which is at a temperature substantially above the plastic temperature of steel, and the other surface at a lower temperature, as high as is possible to maintain with safety, while simultaneously effecting heat transfer through the wall of the said tube or the like at a rate which prevents the plasticizing and pressure deformation of the said tubes. As the steam pressure increases and the temperature gradient between the heat source and the steam zone decreases, it becomes imperative that heat transfer be efficient and rapid lest, failing of such transfer, the heat exchange unit structure becomes overheated and suffers plastic deformation and eventually, perhaps, rupture. In this situation, the deposit, on the water or steam side, of minerals in an insulating layer which impede heat transfer, even modestly, is intolerable.
The problem is particularly exaggerated in the instance of boilers used to supply steam to installations which may return no condensate to the boiler, so that all incoming water is new, or "make-up" water in the sense that it has not previously been, in effect, distilled by passage through the boiler. In this situation, there tends to be a gradual accumulation of minerals capable of forming intolerable deposits.
The removal of minerals from water to be used in boilers has been carefully studied. Installations making use of all of the best available techniques typically begin by treatment of the incoming untreated water with any of numerous flocculating or "settling" agents whereby to remove from the water substantial quantities of turbidity impurities, and leave a relatively clean water. This clean water is typically passed through filters to remove all possible suspended or settling particles. The filtered water may then be deionized. However, various serious problems persist despite all these processes. Thus, clay as a colloid can pass unmodified through all these processes, and at boiler temperatures it tends to break up, yielding within the boiler calcium++, magnesium++, aluminum+++, and silicate--ions in the boiler water. Further, these ions recombine under boiler conditions yielding various mineral substances.
The problem has been mitigated to some extent by the employment of known chemical additives. Various phosphates, any of which, under boiler conditions, becomes converted to an ortho phosphate of some kind, are added because they tend to form complexes with calcium and maintain it in some form in the nature of a suspension. However, when a phosphate is employed alone and at pH of about 9 to 10.5, protection from calcium may be achieved but a tenacious adhesive magnesium phosphate tends to form and develop an insulating coating on the interior (water) surface of the boiler tubes in an intolerable degree.
Magnesium as a problem is mitigated to some extent by inclusion of sodium hydroxide in the boiler water. At a pH which may be in the neighborhood of 11, the hydroxide ion is sufficiently abundant that magnesium predominantly forms magnesium hydroxide, while silica and aluminum are maintained in the form of silicates and aluminates.
With these additions, the problem can be further mitigated by "blow down," that is to say, periodical or continuous removal from the boiler water, at a point of maximum concentration of solids, of a small part of the boiler water whereby some of the mineral components are removed.
A further and related problem of boiler maintenance has been the corrosion of iron and steel boiler tubes by oxygen in the presence of water. Various efforts are made to remove oxygen from the boiler water. In one method that is standard practice in large industrial boilers, the water, before admission to the boiler, is passed into a vessel whereinto live steam is injected under modest pressure. The water temperature is elevated whereby the solubility of oxygen is reduced, and the steam, in passing through, sweeps from the water most of the dissolved gases formerly present. However, after this treatment has been carried forward exhaustively, damaging traces of oxygen usually remain. These traces are frequently "scavenged" chemically. This is accomplished by adding to the boiler water a chemical which will react with the oxygen to bind it chemically and obtain a resulting reaction product that is harmless, or at least less harmful than the uncombined oxygen. The two most commonly used such substances are hydrazine and, preferably, sodium or other alkali metal sulfite. These substances, such as sulfite, are typically added as concentrated aqueous solution to the deaerator. Other chemicals are added through a "chemical feed line" whereby this and other chemical substances previously mentioned are introduced in more or less uniform concentration into the steam drum where the agitation of ebullition quickly disperses them uniformly into the boiler water. Also, sometimes, the oxygen scavenging chemical is introduced into the make-up water feed line to the boiler after steam deaeration, as noted.
Recently, in the attempt to solve the remaining problem of mineral deposits, chelating agents have been included in the chemical feed to the boiler steam drum in the hope that, by reaction with metallic ions, they would form water-soluble organometallic chelates of the objectionable mineral substances. The most widely used and most successful has been the tetrasodium salt of ethylenediaminetetraacetic acid (EDTANa.sub.4). The inclusion of this substance in the chemical feed line through which are added also, typically, sodium hydroxide and a phosphate, has yielded good results in terms of protection of boiler interiors from formation of mineral deposits.
A later method for removing and inhibiting scale is disclosed in U.S. Pat. No. 3,549,538 wherein the treatment employs in combination a nitrilo compound, e.g. the trisodium salt of nitrilotriacetic acid (NTA), and a water soluble polar addition polymer described as sulfoxy-free, e.g. polyacrylic acid or a maleic anhydridestyrene copolymer. Another patent (U.S. Pat. No. 3,492,240) employs a partially hydrolyzed polymer of polyacrylonitrile. A preferred amount of 50-80% carboxyl groups in the hydrolyzed polymer is taught. A more recent patent, U.S. Pat. No. 4,306,991 claims a combination of a copolymer of styrene sulfonic acid and maleic anhydride together with a water soluble organophosphonic acid compound, e.g. methylene diphosphonic acid.
Other amine polycarboxylic acid salts which are good chelating agents have also been suggested for use in boiler water scale inhibition. An important criterion in their selection is the stability of their chelates under the high temperature and pressure conditions found in boiler applications.
Boiler water is frequently treated with an oxygen scavenging compound and with compounds to control pH. The present invention relates to treatment of boiler water which is treated with such an oxygenscavenging compound, especially those compounds which are volatile such as hydrazine. Ammonia, another volatile compound, is added for pH control. The use of such volatile compounds is referred to as AVT (all volatile treatment). Another type of treatment different from the AVT process and alluded to above is that in which phosphates are used for pH control, sometimes in conjunction with ammonia. These are used with sulfite as an oxygen scavenger and with chelants to help control scale and corrosion.
It has been noted in the literature* that the thermal stability of a metal chelate was directly related to its formation constants. The log of the formation constants of Fe(II).NTA, Fe(II).HEDTA**, and Fe(II).EDTA are 8.8, 12.2, and 14.3, respectively. Thus, it was unexpected that HEDTA would exhibit 10 times the thermal stability of EDTA, and thus was effective and superior to EDTA for the prevention of scale in boilers. FNT *Ramunas J. Motekaitis, X. B. Cox, III, Patrick Taylor, Arthur E. Martell, Brad Miles, T. J. Tvedt, Jr,; Canadian Journal of Chemistry; Volume 60, Number 10, 1982. FNT **N-(2-Hydroxyethyl)ethylenediaminetriacetic acid